Percussion drill control means

ABSTRACT

The impact delivered to the bit of a fluid-powered hammer-type drill, especially a down-the-hole rock drill, is automatically adjusted in accordance with the load on the bit so as to maintain maximum drilling efficiency when the bit is operating under both full and part loads. To this end, the exhaust of working fluid from the cylinder spaces of the hammer piston is provided by ports in a central tube which moves with the bit in response to changing loads. The movement of the tube can be arranged to decrease and finally shut off power to the hammer as the bit is progressively unloaded.

Dec. 9, 1975 United States Patent 1191 Shaw 3,193,024 7/1965 Cleary.................,............. 173/73 X 3,503,459 3/1970 Schinoler.,............................ 173/80 PERCUSSION DRILL CONTROL MEANS [75] Inventor: Alan Shaw, Halifax, England Assignee: Halifax Tool p y Limited, Primary ExaminerWi11iam F. Pate, 111 Hallfax, England Assistant Examinerwilliam f. Pate, 111

Dec. 5, 1973 Attorney, Agent, or Firm Kurt Kelman [22] Filed:

21 Appl. No.: 422,068

, ABSTRACT The impact delivered to the bit of a fluid-powered hammer-type drill, especially a down-the-hole rock [30] Foreign Application Priority Data Jan. 9, 1973 United Kingdom................. 1142/73 (11111, is automatically ad usted in accordance with the [52] us CL load on the bit so as to maintain maximum drilling efficiency when the bit is operating under both full and part loads. To this end, the exhaust of working fluid 173/80; 173/115 E21B 5/00; E21C 7/00 [51] Int. 58 Field of 173/13, 15-17, from the cylmder Spaces of the hammer P18101135 173/73, 77, 80, 133, 115, 64 137 58 65; vided by ports in a central tube which moves with the bit in response to changing loads. The movement of the tube can be arranged to decrease and finally shut off power to the hammer as the bit is progressively unloaded.

References Cited UNITED STATES PATENTS 2,917,025 Dulaney..........................., 173/80 x 7 Claims, 1 Drawing Figure PERCUSSION DRILL CONTROL MEANS FIELD OF THE INVENTION The invention concerns percussion drill control BACKGROUND TO THE INVENTION The pneumatically-operated down-the-hole type of.

percussion drill comprises a drill body within which a hammer piston is reciprocable by compressed air fed to the drill body via a string of one or more drill tubes c'onnected to the rear end of the drill body, the hammer piston delivering blows upon the rear end of the shank of a bit which is held in a chuck at the front end of the' drill body so as to be capable of limited axial movement with respect thereto whereby the impact of the hammer piston blows upon the drill bit shank is not transmitted to the drill body and thus dissipated uselessly. The hammer piston is reciprocated under the control of a distributor valve which directs compressed air alternatelyto front and rear cylinder spaces in the drill body, these cylinder spaces also having exhaust ports for dischargeof exhaust air from one cylinder space while compressed air is being admitted to the other.

In some designs of these drills, the exhaust ports are in the walls of the drill body and are thus fixed in position with respect to the latter and since, in theusual case, the ope ration of the distributor valve is controlled by movement of the hammer piston past these exhaust ports, the tendency is for the hammer piston to be reciprocated with maximum efficiency only when its stroke extends between fixed positions in the drill body. However the forward stroke-end position of the hammer piston is determined by the position of contact between the hammer piston and the drill'bit'shank and, especially when drilling in soft and broken ground, the drill bit may take up variable positions relative to the drill body by axial movement within the chuck.

If, as is usual, the distributor valve and the exhaust ports are so arranged as to provide maximum efficiency when the hammer piston is operating between the stroke limits which exist when the bit is fully loaded and at its reamiost position in the drillbody, movement of the bit forwardly relatively to the drill body will cause the stroke of the hammer piston to departfrom its ideal extent and location relatively to the drill body and the operation to become less efficient. One of the consequences of this decrease in operational efficiency" may be that the hammer piston achieves too high an energy in its forward stroke and delivers an excessive blow to the drill bit shank. Moreover, if the drill bit shank reaches the end of its permitted forward movement with respect to the drill body so that there is a mechanical connection effective between the drill bit and the body, the hammer piston blows may be transmitted to the body and cause damage thereto. i

Furthermore, if the hammer piston commences its return, rearward, stroke from aforwardposition beyond its ideal forward stroke end, it may be accelerated to a velocity such that it overshoots its rearmost permissibleposition in the drill bodyand perhaps deliver a blow on the rear end of the rear cylinder space, causing damage to the piston and/ortothe drill body.

Sometimes down-the-hole drills of this general construction have a central tubetby means of which flush-. ing fluid is conveyed to flushing fluid passagesin the. drill bit. In some designs of drills having this central tube, the exhaust ports are provided therein so that the exhaust air is conveyed to the drill bit flushing passages and is used as flushing fluid. However, in known drills having a central tube with exhaust ports therein the tube is fixed in position with respect to the drill body so that the same operational disadvantages arise from fixed location of the exhaust ports as in the case of those designs in which the exhaust ports are in the wall of the drill body.

.It isalso common in down-the-hole percussion drills to provide for automatic cut-off of the hammer operation in the event of the drill bit becoming unloaded, e.g. as a result of drillingthrough broken or faulty ground. A common arrangement is for the drill body to include a rear shock absorber section movable relatively to the remainder oftthe drill body andconnected to the string of drill tubes through which pressure fluid is passed to the drill body, relative movement between the shock absorber section and the remainder of the drill body being effective to operate ashut-off. valve -when the shock absorber section moves relatively away from the remainder of the drill body as a result of unloading of the bit. In certain drills' having this feature, the shut-off valve is in the form of a by-pass valve which opens tostarve the distributor valve of fluid by venting the pressure fluid supply to the exhaust ports and/or to drill bit flushing passages.

Similar constructional and operating principles apply to fluid-powered percussion drills of other than the down-the-hole type.

THE INVENTION b An object of the present invention is to provide improved control means for a percussion drill having a fluid-powered reciprocable hammer piston which delivers blows to the end of a drill bit shank or drill steel shank having limited axial freedom in a drill body, whereby the disc'usseddisadvantages of fixed exhaust port locations are avoided andshut-off valve construction and operation is simplified. t

' In accordance with the invention, therefore, a percussion drill having a fluidpressure-operated' reciprocable hammer piston adapted to deliver blows to the shank of a drill bit or drill-steel having limited a'xial freedom with respect to a drill body includes a central tube adapted to follow at least to a limited extent the axial movement of the said shank with respect to the drill body, said tube having exhaust ports communicating with" cylinder spaces associated with the hammer piston and cooperative with the latter to control the reciprocation thereof.

In preferred embodiments of the invention,said central tube communicates with an exhaust passage formed in the shank of thedrill bit or drill steel as the case may be, this exhaust passage communicating with a bit flushing passage. Alternatively the exhaust passage may communicate with a suitable fluid outlet.

Because the exhaust ports in the central tube cooperate with the hammer piston to control the reciprocation thereof, the stroke of the hammer piston within the drill bodywill be determined by the position of the central tube relatively to the drill body; therefore because the central tube follows the axial movement of the drill bit or drill steel relatively to the drill body it is possible to arrange for the hammer piston to operate with maximum efficiency and deliver substantially the same impact upon the shank of the drill bit or drill steel as the case may be, over at least a substantial part of the permitted range of movement of the said shank relatively to the drill body.

The movement of the central tube with the shank of the drill bit or drill steel relatively to the drill body may also be adapted to control a shut-off valve for the pressure fluid supply to the hammer piston. Thus it may conveniently be arranged that upon forward movement of the said central tube, with the bit or drill steel, to a selected extent the shut-off valve operates to by-pass the hammer by venting the pressure fluid supply to the exhaust outlet or flushing passages as the case may be.

In a preferred embodiment of the invention, a tubular bush at the rear end of the said central tube cooperates with a fixed spigot in such manner that the clearance between the spigot and the bore of the bush varies in accordance with the longitudinal position of the tube within the drill body, the space surrounding this rear end of the tube and the said spigot communicating with the pressure fluid supply upstream of the hammer so that as the central tube moves forwardly the pressure fluid supply is permitted to vent to an increasing extent to the bore of the central tube which thus serves as a by-pass for the hammer. Thus, by choice of an appropriate configuration for the said bush and its cooperating spigot, it may be arranged that the opening of this hammer by-pass occurs progressively as the central tube moves forward, whereby the power available to operate the hammer piston is progressively reduced as the bit or drill steel shank moves forwardly relatively to the drill body to a position where the by-pass is open to an extent leaving insufficient power available to continue to operate the hammer piston.

The central tube may be arranged to follow the entire permitted movement of the drill bit or steel or it may have a more limited movement than the latter, there being a lost-motion connection between the tube and the shank of the bit or steel so as to permit the shank to move more forwardly in the drill body than the tube: with this arrangement, the extra movement of the drill bit or steel when fully unloaded, as compared with the maximum permitted forward movement of the central tube, may be arranged to uncover drain ports in the tube through which liquid accumulations'within the front cylinder space may be discharged.

THE DRAWING The single FIGURE of the accompanying drawing is a partly schematic axial sectional view of a pneumatically-operated down-the-hole percussion drill embodying the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS The drill illustrated in the accompanying drawing is a preferred embodiment of the invention and comprises a body 1 fitted with an internal liner 2 within which a hammer piston 3 is reciprocable. At its front end the drill body 1 is fitted with a detachable chuck 4 which retains the shank 5 of a drill bit 6 provided with flushing fluid passages 7 communicating with an exhasut passage constituted by a central bore 8 in the shank 5. The shank 5 is formed with splines 9 which cooperate with mating splines in the chuck 4 so that torque may be transmitted between the chuck and the drill bit whilst the latter has limited freedom of axial movement within the chuck. At its rear end, the shank has an interrupted peripheral rib 10 constituted by the terminal portions of the splines 9 which are relieved for a distance in front of this rib 10 to provide a circular section length of shank. A diametrally split retaining ring 11 encircles this section of the shank 5, being assembled about this shank section and inserted therewith into the drill body so as to be retained in position to determine the limits of axial freedom of the bit in the chuck by abutment with the rearward ends of the splines 9 or the rib 10.

At the rear end of the liner 2 there is a distributor valve assembly 12 providing on its forward face inlet ports 13 for a rear cylinder space defined by the interior of the liner 2 behind the piston 3; the valve assembly also has lateral ports 14 communicating with longitudinal grooves 15 which in turn communicate with a clearance space 16, between the inner wall of the body 1 and a suitably relieved portion of the liner 2, leading to inlet ports 17 near the front end of the liner. The front end of the piston 3 is of reduced diameter so that the ports 17 in the liner are normally open to a front cylinder space defined by the liner in front of the piston 3 and to the rear of the chuck-4.

The rear face of the distributor valve assembly. 12 has an air inlet port 18 leading to an annular chamber 19 surrounding an annular valve plate 20 movable between the forward position shown, in which it provides communication between the chamber 19 and the ports 14 (and thus with the inlet ports 17 of the front cylinder space), and a rearward position in which it provides communication between the chamber 19 and the inlet ports 13 of the rear cylinder space. The valve disc 20 is oscillated between its two positions by changes in the pressure differential between the front and rear cylinder spaces as determined by the position of the piston 3 within the liner 2.

The piston 3 and the valve assembly 12 are both of annular construction and a central tube 21 extends through both the piston and the valve assembly so as to be axially movable with respect to each. The front end of the tube 21 fits in a counterbore at the rear of the central bore 8 of the bit 6 and has a series of drain holes 22 which are normally disposed within the bore 8 as shown. 1 i

The central tube 8 is desirably formed of a plastics material of high resilience, low coefficient of friction and density, and has two sets of exhaust ports 23, 24 communicating respectively with the front and rear cylinder spaces when the piston 3 is suitably positioned.

The operation of the illustrated distributor valve assembly 12 and its control by movement of the piston 3 relatively to the exhaust ports 23, 24 is well understood in the art. Thus, with the parts in the position shown the piston 3 covers the front exhaust ports 23 while the valve plate 20 of the assembly 12 directs compressed air to the front cylinder space through the inlet ports 17. The piston is thus caused to move rearwardly and in so doing uncovers the ports 23 and covers the ports 24. As a result, the rear cylinder space is sealed so that the moving piston compresses air trapped therein, while air in the front cylinder space is permitted to exhaust into the central tube and so lower the pressure in the front cylinder space. As the piston movement continues, therefore, the pressure at ports 13 of the valve assembly 12 rises above that at ports 14 and causes rearwards movement of the valve plate 20 so as to cut off the ports 17 from the compressed air supply and to direct this instead to the rear cylinder space via inlet ports 13, thereby to cause the piston 3 to be arrested and then impelled forwardly. In like manner, the forward moving piston covers ports 23 and uncovers ports 24 to reverse the pressure differential in the cylinder spaces and return the valve plate 20 to its forward position for repetition of the described operating cycle when the piston has been arrested by impact with the rear end of the shank 5 of the bit 6.

Air exhausted from the cylinder spaces through ports 23 and 24 into the central tube 21 flows to the central bore 8 and thence to the flushing passages 7 of the drill bit 6 to act as flushing fluid to assist in flushing debris from the bore hole and in cooling of the cutting edges of the bit 6.

It will be understood from the foregoing description of the operating cycle of the piston 3 that it is the move ment of the piston 3 relatively to the ports 23 and 24 which effects the variations in the pressure differential between the front and rear cylinder spaces to actuate the valve plate 20 of the distributor valve assembly 12. Thus the ports 23 and 24 are so located in the length of the central tube 21 that, when the front end of the latter is fully seated in the counterbore of the bore 8, the piston reciprocates with maximum efficiency to deliver, at the end of each forward stroke, a blow of desired intensity to the rear end of the bit shank 5.

Because the tube 21 is axially movable with respect to the valve assembly 12 it may follow the movement of the bit 6 from the position shown in the drawing towards the fully forward position of the bit in which the rib engages the locking ring 11. Thus, during forward movement of the bit 6 relatively to the drill body 1, the realtionship of the ports 23, 24 to the struck end of the bit shank 5 is maintained, thereby relating the stroke of the piston 3 to the position of the shank 5 so as to maintain the operation at maximum efficiency.

However, as the bit 6 moves forwardly relatively to the drill body and permits the piston 3 to perform its stroke between correspondingly advanced positions in the body, the forward stroke end position shifts relatively to the inlet ports 17 in the liner 2. Thus as the forward stroke end position advances, the ports 17 become covered to a progressively increasing extent by the piston leading end at such stroke-end position: the effect of this is to trap air in the front end of the bore, to an increasing extent, as the piston approaches its stroke end position and so produce a pneumatic cushion absorbing some of the piston energy and reducing the impact on the shank 5. The risk of the bit being impelled to the end of its permitted forward motion in the chuck is thus much reduced.

In this embodiment of the invention, the permitted axial movement of the tube 21 is less than that permitted to the bit 6, the difference between the permitted axial movements of the bit 6 and the tube 21 respectively being such that when both have moved forwardly to the maximum permitted extent, the tube 21 is effectively withdrawn from the bore 8 in the bit shank 5 to an extent sufficient just to uncover the drain ports 22 in the tube 21. Such a relative positioning of the parts would normally only be attained with the drill body 1 substantially vertical and suspended by its upper (rear) end and in this attitude any liquid in the front cylinder space would thus be able to drain therefrom via the ports 22 and the bore 8 and passages 7.

In order to ensure that the central tube 21 follows any forward movement of the bit shank 5 within the permitted range of movement of the tube 21, the rear end of the latter is fitted with a bush 25, also conveniently formed of plastics material, having a rear surface 26 exposed to the compressed air supply in an annular chamber 27 upstream of the distributor valve assembly 12. Since the front face of the bush 25 is ordinarily exposed only to the approximately atmospheric pressure within the tube 21 there will accordingly be a pressure differential acting on the bush 25 tending to displace the tube 21 forwardly within the drill body. 1, so tending to keep the forward end of the tube 21 seated in the counterbore in the rear end of the bit shank 5.

The bush 25 also forms part of a shut-off valve assembly which includes a spigot 28 extending forwardly from a spider ring 29 fitted in an inlet passage 30 in an adapter coupling 31 which is secured to the rear end of the drill body 1 and to which a drill tube supplying compressed air from a suitable surface rig is connected.

The spigot 28 is of plain cylindrical configuration whilst the bush 25 has a bore the front part 32 of which is cylindrical, to match the spigot 28, and the rear part 33 of which is tapered.

As previously noted, the bit is shown in its rearward most position in the drawing and thus the drawing also shows the tube 21 in its rearward most position. In this position of the tube 21 the spigot 28 fits within the cylindrical bore portion 32 of the bush 25 and thus seals the bore from the compressed air supply in the chamber 27. However, when the tube 21 moves forwardly with the bit 6 relatively to the drill body 1, the bush 25 is progressively drawn from the spigot 28 until the tip of the spigot 28 commences to enter the tapered section 33 of the bore of the bush. At this point, a leakage path exists for compressed air from the chamber 27 to the tube 21 so that a proportion of the compressed air supplied to the drill by-passes the valve assembly 12 and the cylinder spaces within which the hammer piston 3 operates, reducing the power available to maintain operation of the hammer piston.

Thus although the advance of the central tube 21 with the bit 6 will maintairrthe relationship of the exhaust ports 23, 24 to the struck end of the bit shank 5 and thus maintain the hammer piston operating with maximum efficiency relatively to the struck end of the bit to make best use of thefluid power available to the hammer, the available power to the hammer will be progressively reduced as thebit 6 and tube 21 conjointly advance beyond a selected position in their permitted forward movementwith respect to the drill body 1, until eventually so much air will by-pass the hammer by flowing to the tube 21 as to reduce the residual power available to the hammer to a level insufficient to maintain operation.

Accordingly, when drilling with the illustrated drill, if the bit 6 moves forwardly relatively to the drill body 1 for example as a result of the intermittent unloading which can occur when drilling in soft or broken ground, the blows delivered to the bit shank will be reduced and additional air supplied to the bore 8 for use as flushing fluid, as is desirable in these circumstances. In the event of the bit 6 breaking through a fault, for instance and so moving to its extreme forward position, the hammer operation will be interrupted until the drill body 1 has correspondingly advanced in the bore hole and restored the normal load upon the bit, whilst during the interruption of operation of the hammer the compressed air supply will be utilized for flushing the bore hole and preventing jamming of the bit by accumulated debris.

A non-return valve assembly is preferably fitted in the passage 30 to hold air pressurewithin the drill body and cylinder spaces downstream of the valve when compressed air is not being fed to the drill, eg when adding a drill tube to the string connected to the adapter coupling 31, thereby to prevent or reduce backflow of water and/or other debris. Such a nonreturn valve assembly is especially desirable in the case of embodiments in which the central tube 8 is not provided with drain ports 22 and is free to follow the entire permitted forward movement of the bit shank in the chuck 4. In the illustrated embodiment, the non-return valve assembly is disposed between the spider ring 29 and the passage 30 comprising a sleeve 34 fitting in an enlargement of the passage 30 and having a flange 35 at its upstream (rear) end formed with a port 36 surrounded on the downstream face of the flange 35 by a raised annular valve seat 37. A poppet valve 38 with a tubular stem 39 is guided on a spigot 40 on the rear face of the spider ring 29 and is urged rearwardly by a spring 41 to close the port 36 when there is insufficient pressure differential acting on the valve 38 to overpower the spring 41.

I claim:

1. A percussion drill which comprises in combination:

a. a drill body formed internally with a cylinder space;

b. means for securing the shank of a drill bit within the drill body, said means permitting limited axial freedom;

0. a fluid pressure-operated hammer piston reciprocably disposed in said cylinder space to deliver blows to said shank;

d. a central tube extending axially through a distributor valve assembly near one end of the tube and axially through said hammer piston to a central bore of the shank of said drill bit, said central tube having limited freedom for axial movement with said shank;

e. a bush with central bore disposed in said one end of the central tube, the rear surface of the bush being exposed to a supply of compressed air, the front face of the bush ordinarily exposed to pressure within the central tube;

f. a spigot extending forwardly and disposed in an inlet passage for compressed air within the rear end of the drill body, said spigot disposed as a shut-off valve assembly for air within the central tube when in mating contact within the bush; and

g. exhaust port means in said central tube positioned for communication with said cylinder space and cooperative with said hammer piston to control the reciprocation thereof, the central tube communicating with an exhaust passage in said shank and extending to a bit flushing passage h. whereby cooperation of the fixed spigot in said body with said bush providing a clearance between the spigot and bore of the bush which varies in accordance with the longitudinal position of the tube within the drill body, a space surrounding the said rear end of said central tube and the said spigot communicating with a pressure fluid supply passage upstream of the hammer piston for venting pressure fluid to the bore of the central tube upon forward movement thereof.

2. The percussion drill of claim 1, including a lostmotion connection between said central tube and said shank of the bit or drill steel, the axial freedom of said tube being less than the permitted movement of said shank relatively to the drill body.

3. The percussion drill of claim 2, in which said central tube has drain ports therein positioned normally to be covered by the shank of the drill bit, movement of said shank relatively to the drill body in response to unloading of the bit causing relative movement between the said shank and said central tube to uncover said drain ports.

4. The percussion drill of claim 1, including a nonreturn valve in a pressure fluid supply passage therein.

5. The percussion drill of claim 1, in which forward movement of the said central tube to a selected extent operates the shut-off valve to by-pass the hammer by venting the pressure fluid supply to an exhaust passage.

6. The percussion drill of claim 1, including a tubular bush at the rear end of the said central tube a fixed spigot in said body cooperating with said bush and providing a clearance between the spigot and the bore of the bush which varies in accordance with the longitudinal position of the tube within the drill body, a space surrounding the said rear end of said central tube and the said spigot communicating with a pressure fluid supply passage upstream of the hammer piston for venting pressure fluid to the bore of the central tube upon forward movement thereof.

7. A percussion drill which comprises in combination:

a. a drill body formed internally with a cylinder space;

b. means for securing a shank of a drill bit within the drill body, said means permitting limited axial freedom;

c. a fluid pressure-operated hammer piston reciprocably disposed in said cylinder space to deliver blows to said shank;

d. a central tube extending axially through the cylinder space and axially through said hammer piston to a central bore of the shank of said drill bit, said central tube having limited freedom for axial movement less than that permitted to the bit under normal operating conditions;

e. a distributor valve assembly near one end of the central tube and disposed to provide fluid to said fluid pressure-operated hammer piston;

f. one rear end of the central tube being exposed to a supply of compressed air upstream of the distributor valve;

g. a valve assembly disposed between the said rear end and supply of compressed air, said valve assembly permitting compressed air to pass to the distributor valve under normal operating conditions, and to the central tube when the drill bit and central tube move to an extended position beyond normal operating conditions and; 1

h. exhaust port means in said central tube positioned for communication with said cylinder space and cooperative with said hammer piston to control the reciprocation thereof, the central tube communicating with an exhaust passage in said shank and extending to a bit flushing passage. 

1. A percussion drill which comprises in combination: a. a drill body formed internally with a cylinder space; b. means for securing the shank of a drill bit within the drill body, said means permitting limited axial freedom; c. a fluid pressure-operated hammer piston reciprocably disposed in said cylinder space to deliver blows to said shank; d. a central tube extending axially through a distributor valve assembly near one end of the tube and axially through said hammer piston to a central bore of the shank of said drill bit, said central tube having limited freedom for axial movement with said shank; e. a bush with central bore disposed in said one end of the central tube, the rear surface of the bush being exposed to a supply of compressed air, the front face of the bush ordinarily exposed to pressure within the central tube; f. a spigot extending forwardly and disposed in an inlet passage for compressed air within the rear end of the drill body, said spigot disposed as a shut-off valve assembly for air within the central tube when in mating contact within the bush; and g. exhaust port means in said central tube positioned for communication with said cylinder space and cooperative with said hammer piston to control the reciprocation thereof, the central tube communicating with an exhaust passage in said shank and extending to a bit flushing passage h. whereby cooperation of the fixed spigot in said body with said bush providing a clearance between the spigot and bore of the bush which varies in accordance with the longitudinal position of the tube within the drill body, a space surrounding the said rear end of said central tube and the said spigot communicating with a pressure fluid supply passage upstream of the hammer piston for venting pressure fluid to the bore of the central tube upon forward movement thereof.
 2. The percussion drill of claim 1, including a lost-motion connection between said central tube and said shank of the bit or drill steel, the axial freedom of said tube being less than the permitted movement of said shank relatively to the drill body.
 3. The percussion drill of claim 2, in which said central tube has drain ports therein positioned normally to be covered by the shank of the drill bit, movement of said shank relatively to the drill body in response to unloading of the bit causing relative movement between the said shank and said central tube to uncover said drain ports.
 4. The percussion drill of claim 1, including a non-return valve in a pressure fluid supply passage therein.
 5. The percussion drill of claim 1, in which forward movement of the said central tube to a selected extent operates the shut-off valve to by-pass the hammer by venting the pressure fluid supply to an exhaust passage.
 6. The percussion drill of claim 1, including a tubular bush at the rear end of the said central tube a fixed spigot in said body cooperating with said bush and providing a clearance between the spigot and the bore of the bush which varies in accordance with the longitudinal position of the tube within the drill body, a space surrounding the said rear end of said central tube and the said spigot communicating with a pressure fluid supply passage upstream of the hammer piston for venting pressure fluid to the bore of the central tube upon forward movement thereof.
 7. A percussion drill which comprises in combination: a. a drill body formed internally with a cylinder space; b. means for securing a shank of a drill bit within the drill body, said means permitting limited axial freedom; c. a fluid pressure-operated hammer piston reciprocably disposed in said cylinder space to deliver blows to said shank; d. a central tube extending axially through the cylinder space and axially through said hammer piston to a central bore of the shank of said drill bit, said central tube having limited freedom for axial movement less than that permitted to the bit under normal operating conditions; e. a distributor valve assembly near one end of the central tube and disposed to provide fluid to said fluid pressure-operated hammer piston; f. one rear end of the central tube being exposed to a supply of compressed air upstream of the distributor valve; g. a valve assembly disposed between the said rear end and supply of compressed aiR, said valve assembly permitting compressed air to pass to the distributor valve under normal operating conditions, and to the central tube when the drill bit and central tube move to an extended position beyond normal operating conditions and; h. exhaust port means in said central tube positioned for communication with said cylinder space and cooperative with said hammer piston to control the reciprocation thereof, the central tube communicating with an exhaust passage in said shank and extending to a bit flushing passage. 